proc dial {w var min max diameter bg fg mg} {
    # create the dial canvas
    canvas $w -width $diameter -height $diameter -bg $bg \
        -highlightthickness 0
    # create a data array
    upvar \#0 $w data
    # cleanup the data array
    bind $w <Destroy> [list unset $w]
    # save arguments
    foreach v {w var min max diameter bg fg mg} {
        set data($v) [set $v]
    }
    # compute some values
    set data(x0) [expr {$diameter/2}]
    set data(y0) [expr {$diameter/2}]
    set data(r0) [expr {($diameter-10)/2}]
    set data(dr) [expr {$data(r0)/5}]
    set data(mark-theta) 250
    set data(2pi) [expr {2*atan2(0,-1)}]
    # create the dial disk
    $w create oval 5 5 [expr {$diameter-5}] [expr {$diameter-5}] \
        -fill $fg -outline $mg -width 4 \
        -tags dial
    # mark the resolution radii with concentric circles
    foreach d {2 3 4} {
        set r [expr {$d*$data(dr)}]
        $w create oval \
            [expr {$data(x0)-$r}] [expr {$data(y0)-$r}] \
            [expr {$data(x0)+$r}] [expr {$data(y0)+$r}] \
            -fill {} -outline $mg -width 4 \
            -tags dial
    }
    # draw a rotating marker
    $w create line $data(x0) $data(y0) $data(x0) 5 \
        -width 4 -fill $mg -arrow last \
        -tags {dial mark}
    # press binding
    $w bind dial <ButtonPress-1> {dial-press %W %x %y}
    # motion binding
    $w bind dial <B1-Motion> {dial-motion %W %x %y}
    # return our window
    return $w
 }
 proc dial-press {w x y} {
    # bind to data array
    upvar \#0 $w data
    # determine radius and theta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set data(r) [expr {sqrt($x*$x+$y*$y)}]
    set data(theta) [expr {int(1000*atan2($y,$x)/$data(2pi))}]
    # compute frequency update for this radius
    # this assumes the 1000 counts/rev done below
    switch [expr {int(($data(r0)-$data(r))/$data(dr))}] {
        0 {
            set data(kHz/count) 0.01;   # 10 kHz/rev
        }
        1 {
            set data(kHz/count) 0.1;    # 100 kHz/rev
        }
        2 {
            set data(kHz/count) 1;      # 1 MHz/rev
        }
        default {
            set data(kHz/count) 10;     # 10 MHz/rev
        }
    }
 }
 proc dial-motion {w x y} {
    # bind to data array
    upvar \#0 $w data
    # compute dtheta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set dtheta [expr {(int(1000*atan2($y,$x)/$data(2pi))-$data(theta))%1000}]
    if {$dtheta > 500} { set dtheta [expr {$dtheta-1000}] }
    # update theta
    set data(theta) [expr {($data(theta)+$dtheta)%1000}]
    # update marker
    set data(mark-theta) [expr {$data(mark-theta)+$dtheta}]
    set t [expr {$data(2pi)*$data(mark-theta)/1000}]
    $w coords mark $data(x0) $data(y0) \
        [expr {$data(r0)*cos($t)+$data(x0)}] \
        [expr {$data(y0)-$data(r0)*sin($t)}]
    # update frequency
    upvar \#0 $data(var) frequency
    set f [expr {$frequency-$dtheta*$data(kHz/count)}]
    if {$f < $data(min)} {
        set f $data(min)
    } elseif {$f > $data(max)} {
        set f $data(max)
    }
    # update frequency variable
    set frequency [format %12.2f $f]
 }

 # Here's some demo code:
 set bg \#4f4f4f
 set fg grey
 set mg white
 set frequency 15000.00
 . configure -bg $bg
 pack [frame .f -bg $bg] -side top -fill x
 pack [label .f.l -font {-size 32} -width 9 -anchor e \
        -textvar frequency -bg $bg -fg $mg] -side left
 pack [label .f.hz -font {-size 32} -text "kHz" -bg $bg -fg $mg] -side  left
 pack [dial .d frequency 0.00 30000.0 200 $bg $fg $mg] -side top